Scala 如何在F多态代码中使用guave缓存加载程序

我有一个服务，这是一个笑话：

但我想先用guava cache来缓存请求的笑话（仅通过常量键，这无关紧要）：

object Jokes {
  def apply[F[_]](implicit ev: Jokes[F]): Jokes[F] = ev

  final case class Joke(joke: String) extends AnyRef
  object Joke {
    implicit val jokeDecoder: Decoder[Joke] = deriveDecoder[Joke]
    implicit def jokeEntityDecoder[F[_]: Sync]: EntityDecoder[F, Joke] =
      jsonOf
    implicit val jokeEncoder: Encoder[Joke] = deriveEncoder[Joke]
    implicit def jokeEntityEncoder[F[_]: Applicative]: EntityEncoder[F, Joke] =
      jsonEncoderOf
  }

  final case class JokeError(e: Throwable) extends RuntimeException

  def impl[F[_]: Sync](C: Client[F]): Jokes[F] = new Jokes[F]{

    val cacheLoader : CacheLoader[String, Joke] = new CacheLoader[String, Joke] {
      override def load(key: String): Joke = {
        import dsl._
        val joke: F[Joke] = C.expect[Joke](GET(uri"https://icanhazdadjoke.com/"))
          .adaptError{ case t => JokeError(t)}
        //? F[Joke] => Joke
        null
      }
    }

    val cache = CacheBuilder.newBuilder().build(cacheLoader)

    val dsl = new Http4sClientDsl[F]{}

    def get: F[Jokes.Joke] = {
       //it's ok?
       cache.get("constant").pure[F]
    }
  }
}

如您所见，

cacheLoader

需要“物化”值

F[Joke]=>Joke

。和缓存返回纯值，不带

F

---

如何在

F

polymorpic code中使用此缓存？

您基本上是在询问如何在

F

中运行代码多态性，为此，您需要对

F

进行

效果

约束

此外，您还需要使用

延迟

，而不是使用

纯

，因为从缓存中获取值是一种副作用

val cacheLoader : CacheLoader[String, Joke] = new CacheLoader[String, Joke] {
  override def load(key: String): Joke = {
    import dsl._
    val joke: F[Joke] = C.expect[Joke](GET(uri"https://icanhazdadjoke.com/"))
      .adaptError{ case t => JokeError(t)}

    // This is a side effect, but can't avoid it due to the way the API is designed
    joke.toIO.unsafeRunSync()
  }
}

val cache = CacheBuilder.newBuilder().build(cacheLoader)

val dsl = new Http4sClientDsl[F]{}

def get: F[Jokes.Joke] = {
   // This is okay :)
   Sync[F].delay(cache.get("constant"))
}

另一方面，如果您想使用与http4s能够很好地互操作的东西，我强烈推荐

mules

。在这里查看：

---

For me line

val cache=CacheBuilder.newBuilder（）.build（cacheLoader）

生成

错误：（52,43）推断类型参数[String，io.chrisdavenport.clientexample.Jokes.Jokes.Jokes]不符合方法构建的类型参数界限[K1编辑
扩展AnyVal
=>
扩展AnyRef
。现在编译
val cacheLoader : CacheLoader[String, Joke] = new CacheLoader[String, Joke] {
  override def load(key: String): Joke = {
    import dsl._
    val joke: F[Joke] = C.expect[Joke](GET(uri"https://icanhazdadjoke.com/"))
      .adaptError{ case t => JokeError(t)}

    // This is a side effect, but can't avoid it due to the way the API is designed
    joke.toIO.unsafeRunSync()
  }
}

val cache = CacheBuilder.newBuilder().build(cacheLoader)

val dsl = new Http4sClientDsl[F]{}

def get: F[Jokes.Joke] = {
   // This is okay :)
   Sync[F].delay(cache.get("constant"))
}